The present invention relates to a color cathode ray tube, and more particularly to a color cathode ray tube having an improved slot type shadow mask.
The cathode ray tube presently in wide use is a so-called shadow mask type color cathode ray tube. In a shadow mask type color cathode ray tube, traveling paths of three electron beams corresponding to red, green and glue colors emitted from an electron gun are selected and controlled through the shadow mask so as to impinge only upon respective ones of phosphors of red, green and blue colors which are disposed in discrete portions on the inner surface of a panel. Generally, a shadow mask is constituted by a thin plate of iron or iron-nickel alloy having a small thermal expansion coefficient, the thin plate having electron beam passage apertures which are circular, rectangular or slits formed between a parallel array of narrow strips held together only at the ends. On the inner surface of the panel are formed phosphors in discrete portions as mentioned above and in a form corresponding to the shape of the electron beam passage apertures.
The shadow mask having circular apertures, the one having rectangular apertures and the one having slits formed between a parallel array of narrow strips held together only at the ends are here designated a dot type shadow mask, a slot type shadow mask and a grille type shadow mask, respectively.
The slot type shadow mask can also be considered to have a structure provided with bridges disposed at an interval for connecting parallel grilles of the grille type shadow mask.
In many cases, the dot type shadow mask and the slot type shadow mask have a spherical form, while the grille type shadow mask is held in a cylindrical form with tension applied between its top and bottom.
In shadow masks, the temperature thereof rises, thus causing thermal expansion, due to impingement of electron beams on the shadow mask during operation of the cathode ray tube.
Recently there has been a strong demand for a high definition display on a cathode ray tube, and with the improvement in the resolution of the display, the necessity of suppressing to a higher degree the deviation in landing positions of electron beams on the phosphor screen caused by a shift of the electron beam passage apertures due to a thermal expansion of the shadow mask has become stronger and stronger.
For diminishing the above thermal expansion, there has been used an expensive iron-nickel alloy having a small thermal expansion coefficient as a constituent material of the shadow mask, or the shadow mask has been held on a mask frame under tension.
In the case of the above-mentioned shadow mask held under tension, during the procedure of stretching a grille type shadow mask and fixing it in a cylindrical form on a frame by tensing it in the vertical direction of the mask, the grilles become easily entangled with one another, thus making it difficult to handle the shadow mask.
For solving the above-mentioned problems, it has been attempted to hold a dot type shadow mask or a slot type shadow mask on a mask frame under tension, for example as disclosed in U.S. Pat. No. 4,973,283.
In the case where a slot type shadow mask is held on a frame under the application of tension in the vertical direction of the picture display screen in accordance with the prior art, the entanglement problem of grilles does not occur because of the presence of bridges even when the shadow mask is made thin, thus making it possible to lighten the mask frame. In this case, however, a Poisson contraction, a compressive force generated according to Poisson's ratio and acting in the transverse direction, is propagated to the right and left by the bridges, so that the grilles of the shadow mask are no longer vertically rectilinear but are bent toward the center as the distance from the center increases in the transverse direction.
In the grille type and slot type shadow masks, the portion which fulfills the intrinsic function of color selection is the grille portion.
The slot type shadow mask can also be considered to have a structure provided with bridges disposed at an interval for mechanical connection of parallel grilles with one another, which structure is adopted for eliminating the drawback of the grille type shadow mask wherein the grilles get entangled with each other during fabrication of the mask, causing great inconvenience in the handling of the mask.
The grille type shadow mask is held in a cylindrical form under the application of tension in the vertical direction, while the slot type shadow mask usually has a spherical form.
In the slot type shadow mask, as the definition of the display is improved, it is possible to cope with beam landing errors due to thermal expansion by using an iron-nickel alloy having a small thermal expansion coefficient as the material of the shadow mask. However, this alloy is expensive as mentioned previously.
The slot type shadow mask is formed of iron and if it is held under vertical tension like the grille type shadow mask, there occurs an inconveniences such that the mask frame used is required to have a high rigidity and is therefore heavy.
Further, when the slot type shadow mask is held under the application of vertical tension in parallel with the grilles, a transverse compressive force generated according to Poisson's ratio is propagated to the right and left through the bridges, thus causing an inconvenience such that the grilles of the shadow mask are not vertically rectilinear but are bent toward the center of the mask as the distance from the center increases in the transverse direction. This is as noted previously.